Severe hypophosphataemia: a rare cause of postoperative muscle weakness.

We report a case of severe generalised muscle weakness in a 66-year-old man who underwent revision of left knee arthroplasty. On postoperative day 1, he developed non-focal muscle weakness and shortness of breath which progressed over a 6-hour period. Serum phosphorus level was severely low at 0.5 mg/dL, along with mild degree of hypokalaemia, hypocalcaemia and hypomagnesaemia. His symptoms completely resolved after emergent phosphorus replacement. The authors believe this case is of educational interest to physicians as generalised muscle weakness is an uncommon presentation of severe hypophosphataemia. In a postoperative setting, hypophosphataemia is often multifactorial, thought to result from combination of perioperative catecholamine surge, administration of saline, diuretics, glucose and antacids, poor oral intake and respiratory alkalosis secondary to pain. We report this case to raise awareness among physicians on severe phosphate imbalance as the primary aetiology for acute generalised muscle weakness and respiratory failure, especially after a surgery.